1. Field of the Invention
The present invention relates to a hybridly integrated module which includes at least one ASIC element, one MEMS element, and one cap wafer. The ASIC element is configured with circuit functions which are integrated into the ASIC substrate. The ASIC element furthermore includes a layer structure on the ASIC substrate which includes at least one wiring level for the circuit functions. At least one micromechanical structure component, which extends over the entire thickness of the MEMS substrate, is formed in the MEMS element. The MEMS element is mounted on the layer structure of the ASIC element, so that a gap exists between the micromechanical structure component and the ASIC element. The cap wafer is mounted over the MEMS element in a pressure-tight manner, so that the micromechanical structure component is situated in a sealed cavity between the ASIC element and the cap wafer.
2. Description of the Related Art
The module concept of vertically hybridly integrated modules of the type in question here allows cost-effective mass production of very small, compact and robust modules having a micromechanical function and the associated circuit functions for signal processing. For this purpose, the individual module components—ASIC element, MEMS element and cap—are not only produced on the uncut wafer; their assembly to form a module also generally takes place on the uncut wafer. Moreover, the MEMS functions and the ASIC functions may be tested at the wafer level, and even the balancing of the modules may still be carried out at the wafer level prior to the separation.
To manufacture such a vertically hybridly integrated module, initially the ASIC substrate is processed and provided with a layer structure having wiring levels for the individual circuit functions. Then, the starting substrate for the MEMS element is mounted on this layer structure. The mechanical joint between the MEMS substrate and the ASIC layer structure is generally produced in a direct bonding process, the process temperature having to be kept at a relatively low level, preferably under 500° C., so as not to damage the circuit functions in the ASIC substrate. If the circuit functions of the ASIC element are exposed to higher process temperatures, it is possible that their functionality may no longer be ensured. The MEMS substrate is structured only after it is mounted on the ASIC layer structure to expose the structure components of the micromechanical function. Then the cap wafer is positioned over the micromechanical structure component and is either joined to a frame area of the MEMS substrate or is mounted on the ASIC layer structure, if during the structuring of the MEMS substrate a corresponding circumferential mounting surface was exposed on the ASIC layer structure. In any case, the mounting of the cap wafer must also be carried out at relatively low process temperatures out of consideration for the integrated circuit functions in the ASIC substrate.
In many applications, such as sensor applications, the cap wafer is not only used as mechanical protection, but also to hermetically seal the micromechanical structure components and to implement defined pressure conditions for the micromechanical function of the MEMS element.
In this connection, the direct bonding process for assembling the MEMS substrate and the cap wafer proves to be problematic. This is because at low process temperatures, which are non-critical for the ASIC circuit functions, no permanently pressure-tight joints may be established between the substrates and the layer materials which are typically used as the MEMS substrate and cap wafer and for the ASIC layer structure. For example, during the direct bonding of a silicon substrate and an oxide layer, annealing temperatures of over 1000° C. are required if this joint is to remain pressure-tight over the service life of the module.